This invention relates generally to the reliability testing of conductors that are subject to mass transport of atoms, and, is more particularly directed to a process for evaluating the reliability of a thin-film interconnector of the type typically employed in micro-electronic devices.
Thin-film conductors of the type typically employed in microcircuits and other integrated circuits deteriorate over time due to a process known as electromigration. Under certain adverse conditions, this process can lead to early circuit failure. Electromigration failure generally involves the movement of atoms in the direction of current flow from a donor region into an acceptor region.
Electromigration failure generally occurs as two separate stages. During a first stage of failure, referred to as the electromigration damage (EMD) stage, atoms move out of the donor region under relatively well defined conditions, leaving behind voids in the material. The transported atoms are deposited in the acceptor region thereby creating hillocks. The second stage of electromigration failure, referred to as the catastrophic failure process (CFP) stage, is characterized by complex temperature and current density variations that lead to rapid and complete failure of the device. These two stages of electromigration failure always occur in sequence, with EMD being first. The damage that occurs in the early stages of the process follow well-defined conditions of temperature, temperature distribution, and current density. These conditions remain relatively constant during EMD, and to a great extent control the failure process over the life cycle of the conductor. The second, more dramatic stage of the failure process, while still electromigration is not characterized by the initial conditions of temperature and current density previously experienced by the conductor, but rather by local current densities and temperatures that develop in the now highly stressed donor region. Although the second stage of failure is a consequence of the first, it nevertheless occurs with rapid kinetics and under less well-defined conditions than those experienced during the earlier stages. EMD occurs over a major portion of the conductor life, while CFP takes place during a relatively brief period at the end of the life cycle. That is, EMD controls the overall failure process, and thus lays the template for the ultimate CFP.
During the past years different techniques have been applied to determine the activation energy. The life time test is the most common measurement method which records the mean time to failure (t.sub.50) when 50% of identical samples fail due to electromigration. Mean time to failure is measured as a function of test temperature to evaluate activation energy Q and the pre-exponential A.
The resistance method is the most simple technique. A stripe is stressed at constant current and temperature. The increase in the resistance of the stripe with time is recorded. The activation energy and pre-exponential are determined from the temperature dependence of the resistance change per unit time.
Electrical resistance and resistivity measurements taken under isothermal conditions have been employed to study the kinetics of the electromigration process, and a thorough treatment of this type of testing is given by Hummell et al in Journal of Physics and Chemistry of Solids, Pergamum Press, 1976, Vol. 37, pp 73-80, (printed in Great Britain).
Recently a new technique called Temperature-ramp Resistance Analysis to Characterize Electromigration (TRACE) has been developed. Its principal advantage is that the activation energy and pre-exponential can be determined from one experiment requiring a few hours. This method allows operation through a given temperature range in a pre-selected time. In addition, this method allows systematic investigation of low temperature processes normally ignored by constant-temperature experiments. This process is set forth in U.S. Pat. No. 4,483,629.
While the dynamic testing process of the U.S. Pat. No. 4,483,629 has been highly successful in predicting the life cycle of conductors of complex integrated circuitry, that process requires that the IC devices be finished, that is, encapsulated in a shell or package, and that the devices be tested in the controlled atmosphere of a sealed chamber.
Electromigration phenomenom in thin films are currently receiving considerable attention. This is largely due to the implications of electromigration damage (EMD) on the reliability of Very Large Scale Integration (VLSI) devices.
Until now, all the above techniques have been applied to study the kinetics of the electromigration process in packaged samples. While the life time test and the resistance method have been applied at the wafer level, TRACE experiments have not been tried. The objective of the present invention is to apply TRACE at the wafer level. This objective is not trivial. Due to rapid analysis afforded by the TRACE technique, new metallizations and stripe architecture can be screened directly, eliminating the costly and time consuming steps of packaging.